1. Field of the Invention
This invention pertains generally to the field of power supply circuits and systems, and more particularly to circuits which are capable of controllably transferring and recovering energy at high instantaneous power levels between a power source and a load.
2. Description of the Prior Art
The problem of providing regulated DC voltage to a load from an AC source is a common one, and, of course, numerous power supply systems have been developed. In power supplies for loads which require large amounts of power, it is important to minimize the power that is consumed in losses in the power supply system itself. A common solution to the power supply problem at large power levels is the use of a controlled rectifier bridge which employs thyristors to provide a controlled amount of power to the load. By adjustment of the firing angle of the thyristors, the average voltage supplied to the load may be controlled over reasonable design limits.
The difficulty of supplying repetitive pulsed power loads which are highly inductive is particularly acute. Such inductive loads may demand a large amount of instantaneous power from the power system which is substantially greater than the average power which must be provided by the system. Loads of this nature are typical of the inductive loads that are contemplated for fusion reactors such as the Tokamak reactor, and for present and contemplated particle accelerators and other machines used in high energy physics which employ large electrical magnets.
In a typical controlled rectifier or converter such as a Graetz bridge, the amount of power supplied to an inductive load may be controlled by controlling the delay angle of the firing signal supplied to the gate inputs of the thyristors in the bridge. Such a system may also provide the capability of reversing the flow of power such that power is supplied from the inductor to the power system, depending on the magnitude of the delay angle. While such converters are useful for transmitting power to an inductive load, they give rise to unacceptable voltage dips in the AC power system when the converter is attempting to provide a controlled and regulated voltage to a large inductive load, such as is required when a fusion reactor winding is being charged and discharged. Moreover, the charging of such an inductive load may require large amounts of momentary power from the system over a relatively short period of time, with this power later being returned to the power system when the inductive load is discharged. While the average power demand on the power system may be very small if the controlled rectifier and inductive load are nearly lossless, the large amounts of peak power required (possibly up to 1000 megawatts) from the power system makes such a scheme inefficient and possible infeasible for the very large pulsed power surges that will be required by fusion reactors.
Energy storage devices such as storage batteries and pumped hydro-storage may be utilized under some circumstances to augment the capabilities of the power system for the short pulsed surges of power that are required, and systems of this type have been considered for supplying the daily peak demands of commercial power systems. Large superconductive energy storage inductors coupled to the commercial power system by means of a converter may also be utilized to augment the power system in supplying the pulsed peak power requirements. See e.g. R. W. Boom and H. A. Peterson, "Superconductive Energy Storage for Power Systems," IEEE Transactions on Magnetics, MAG-8, Sept. 1972, pp. 701-703. While such energy storage systems are useful to augment the power supplied from the commercial power system, they are not well adapted to limiting the power system voltage dips when repetitively pulsed power loads of large magnitude are being supplied.
Additional problems are encountered because of the high voltages and high currents encountered when supplying loads which require large amounts of real or reactive power, such as the windings of particle accelerator magnets or proposed fusion magnets. Most power supply systems require a metal core transformer having numerous windings, and transformers of this type which are capable of carrying the high voltages and currents required are expensive and bulky. In addition, such transformers contribute greatly to the overall energy losses in the system.